Plastics Flashcards
what are some of the applications of plastic
construction
packaging
plastic lumber
automotive
what are the advantages of plastic processing
no further shaping is needed
less energy is required
painting is normally not required
handeling of product is simplified
disadvantages of plastics
low strength
low modulus of elasticity
limited useful thermal range
can degrade ehen subjected to radiation
fragile
dimensional instability
flammable
suceptible to chemical attack
may have associated health risk
advantages ofd the plastic material
high corrosion resistance
good insulating properties
good strength to weight ratio
ease of processing and reprocessing
capable of being rigid or flexible
can be transperant, translucent, or opaque
what are the 3 types of polymers
thermoplastics
thermosetting
elastomers
what are thermoplastics
they can be heated, cooled and reheated.
can be recycled
dont crosslink when heated
what are the 3 ways crosslinking is accomplished
tempurature
catalyst
mixing
what are thermosets
undergo curing when heated
crosslinking happens when heated
cannot be remelted
what are elastomers
caple of large elatic deformation
why are thermosets sometimes better then thermoplastics
more rigid
brittle
more resistant to common solvents
capable of higher surface tempuratures
what makes elastomers have the incrediable elatic properties
long molecules
dergree of cross linking is lower then thermosets
what are the 2 molecular structures for polymers
crystalline
amorphous
what is the properies of crystline steructure
not clear
opaque or translucent plastic
chains packed closely together
what is the properies of amprphous steructure
spagetthi type structure
clear plastics
what are the shaping processes for platics
injection molding
extrustion
compresstion molding
blow molding
rotational molding
calendering
composites
what is injection molding
a process wheere a heated polymer is forced into a cavity
injection molding facts
very similar to die casting
complex shapes possible
high tool cost
long lead time
economical for high volume production
typically for thermoplastics
what are the 3 units in the injection molding machine
injection unit
mold
clamping unit
how does the injectioln molding process work
1) mold is closed
2) melted plastic injected in cavity
3) screwback
4) mold opens and part is released
what are the 2 parts of the mold
cavity: internal design
core - external design
why are the 2 halves of the mold called the hot and cold sides
hot side- material is injected though this side
cold side- materials is cold and enjection system is only on this side
what is shrinkage
as the polymes cool they will shrink and the amount of shrinnkage is dependant on the type of plastic
what are the shrinkage factors
injection pressure
compaction time
molding tempurature
fillers tend to reduce shrinkare
what are the 4 common injection molding defects
short shott
flash
sink marks/voids
weld lines/knit lines
what are the causes of short shot
not enough plastic
to cold
to slow to inject
what are the causes of flash
spaces is mold
fhigh injection pressure
melt tempurature to high
mold wearing
what are the causes of sink marks
thick mold sections
what are the causes of weld lines
temp to low
speed to love
what is a die
a tolol used to cut/form material into desired form/shape
what is extrusion of plastic
compression process in which flow of material is forced through a die
what are some applications of extrusion
pipes
hose
structural shapes
sheet/film
continuos filiment
coated elctrical wire
what are the main components of the extruder
barrel
screw
die
what are some common die profiles
solid
hollow
wire/cable coating
sheet/film
filaments
what is viscoelasticty
the shape memory of the material
what is an example of a solid profile
rounds
squares
ireggular cross section
what happens when solid profiles are extruded
material is still soft after die
to mnimize visoelasticy the die is made long
what is an example of a hollow profile
pipes
hoses
any cross section with a hole
what happens when hollow profiles are extruded
mandrel required to form internal shape
what happens when wire/ cable coating is extruded
pulled at high speed
vaccum is drawn to promote adhesion
what is sheet/film made of
film- thermolplastics
sheet- thermorming
what are some applicatoiooons of film
packaging
pool covers
liners for irregation
what are the processes for sheet/film
split die extrusion
blown film extrustion
calendaring
what is split die extrusion
conventional extrusion but uses narrow slit as die opening
uniformity is sometimes a problem
what is blown film extrusion
combines extrusion to produce of thin film
what is the process of blown film extrusion
extrusion of tube
tube is drawn to upward and expnded by air at the same time
air pressure kept constant to keep thickness of film
what is the frost line
marks the position where solidification occurs.
what is calendaring
a process for producing sheet/film out of rubber or rubbery thermoplastics
what is the process of calendaring
feedstock is passed through the series of rolls to reduce thickness to desired games
calendaring facts
expensive equipment
high productiomn rates
has good surface finish and accuracy
what are applications of calendaring
pvc flooring
shower curtians
table cloths
pool liners
inflatable toys
what sis the comppression molding process
1) material is loaded
2) material is compressed and cured
4) part is ejected amd removed
what are the possible molding compounds availble
powders
pellets
liquid
preform
compression molding facts
used with thermosets and elastomers
used for rubber tires an dpmc parts
marterial is preheated to shorten cycle times
mold is heated to help cure the material
compression vs injection molding
compession: molds are simpliar
injection: shorter cycle times and higher production rates
what are the common molding materials in compression molding
eppoxies
urethanes
elatomers
phenolics
melamine
applications for compression molding
electric plugs
pot handles
electrical sockets
dinnerware plates
what is tranfer molding
very closely related to compression molding
what are the 2 different types of transfer molding
pot tranfer molding
plunger transfer molding
what is the basic transfer molding process
1) material sis loded into chamber
2) pressure applied as polymer is pressed into cavity
3)part is ejected
pot vs plunger
pot: injected through vertical sprue channel
plunger: injected through heated well
what is blow molding
a process where air pressure is used to inflate the soft plastic
an important process for making really thin hollow parts
what is the process of blow molding
1) create tube (parson)
2) add heated parison into tube
3) clos the mold
4) add the compressed air into the mold
5) trim product and remove from mold
what are the 3 differnt blow molding procsses
injection
extrusion
stretch
what are the advantages of extrusion blow molding
reduces stresses
has weight reduction properties
reduction on waste
reduced cooling times
whats is speciall about blow molding
only can be used with thermoplatics
what is rotaionial molding
a process that uses gravity inside a rotating mold to form a holllow shape
its a simple but slow process
what polymers can be used in rotational molding
mainly for thermoplastics but can be used for elastomers and thermosets
what is the process of rotational molding
- material is loaded into mold
- mold is closed, heated, and rotated on 2 axies
- plastic is colloed while still spinning
- part is removed
what is thermoforming
flat thermoplastic film/sheet is heated or deformed into desired shape using mold
what is the process of vacumn thermoforming
- plastic is heated
- sooftened platic is placed over negitive mold
3.a vacumn effec draws the sheet into the cavitys - plastic hardens and shape is removed
postive vs negitive molds
positive- inerior matched the countour of the mold
negive- the exterior will math the contour of the mold
what are the general design considerations for plastic
strength/stiffness
impact resistance
service tempuratures
thermal expansion
degradtion
what are the design considerations for extrusion
walol thickness
hollow sections
sharp corners
shrinkage
die swell
what are the design considerations in injection molding
economic quantities
part complexity
wall thickness
corner radii and fillets
ho;es
shrinkage
parting line location
slides/side actions
draft angles
tolerances
